Atrial and ventricular pacer having independent rate and av delay controls

ABSTRACT

There is disclosed an atrial and ventricular pacer having independent rate and AV delay controls. Adjustment of one potentiometer changes the basic pacer rate without affecting the AV delay; adjustment of another potentiometer varies the AV delay without affecting the pacer rate. This is achieved by having the rate control determine the atrial escape interval, a variable one-shot multivibrator being provided to delay the ventricular stimulus after it is triggered simultaneously with the generation of an atrial stimulus. The variable one-shot multivibrator is reset when a spontaneous ventricular beat is detected, and the atrial timing period always begins at the end of a fixed time interval following an atrial stimulus, the fixed time interval being longer than the maximum AV delay.

United States Patent Thaler et a1.

Jan. 8, 1974 ATRIAL AND VENTRICULAR PACER Primary Ex'aminerWilliam E. Kamm HAVING INDEPENDENT RATE AND AV Attorney-William C. Nealon, Bernard L. Sweeney DELAY CONTROLS and Joel Wal [75] Inventors: Sherwood S. Thaler, Lexington; [57] ABSTRACT gg g ggfigg Chelmsford There is disclosed an atrial and ventricular pacer hav- I 'ing independent rate and AV delay controls. Adjust- [73] Assignee: American Optical Corporation, ment of one potentiometer changes the basic pacer southbridge, MaSS- rate without affecting the AV delay; adjustment of an- [22] Filed: 6 1971 other potentiometer varies the AV delay without affecting the pacer rate. This is achieved by having the 1 1 pp N05 205,178 rate control determine the atrial escape interval, a variable one-shot multivibrator being provided to [52] us CL 128/419 P 128/421 delay the ventricular stimulus after it is triggered si- [51 1 Int. Cl 1 1/36 multaneously with the generation of an atrial stimulus. [58] Field 421 422 The variable one-shot multivibrator is reset when a spontaneous ventricular beat is detected, and the l 56] References Cited atrial timing period always begins at the end of a fixed time interval following an atrial stimulus, the fixed UNITED STATES PATENTS time interval being longer than the maximum AV de- 3,595,242 7/1971 Berkovits 128/419 P b 3,593,718 7/1971 Krasner et a1. 128/419 P 7 Claims, 2 Drawing Figures RESET 30 l6 32 RESET E3 7 2 ATRIAL 2 0 MSEC VENTRICULAR 50-250 s c VENTRICULAR PJ/Iil'lk ONE-SHOT J ONE- SHOT T STIMULATO R f 1 E2 E4 t 20 j 4 f 134 T 3 g 4 P-P-RATE CONTROL A OL This invention relates to atrial and ventricular demand pacers', and more particularly to such pacers in which the rate and AV delay controls are independent .of each other;

There are many patients who require an atrial and ventricular pacer, as opposed to the more usual ventricular pacer. in demand pacers of the latter type, a detector monitors the spontaneous ventricular beating of the heart; if too long a time interval has elapsed since the last beat, a stimulating pulse is generated to trigger the ventricular beat. in pacers of the former type, for example, in that disclosed in Berkovits application Ser. No. 884,825 filed on Dec. 15, 1969 and which has matured into US. PatfNo. 3,661,158, an additional circuit is provided for generating atrial stimulating pulses to compensate for irregular atrial activity. To maintain synchronism between the two pulsing circuits, every ventricular beat. whether spontaneous or stimulated causes the atrial timing period to re-start'together with the (longer) ventricular timing period. Following any ventricular beat, an atrial stimulating pulse is generated a short time after the next atrial beat should occur. The atrial stimulating pulse is generated even if a spontaneous atrial beat occurs. Thereafter, a ventricular stimulating pulse is generated, but only if a spontaneous ventricular beat does not'occur within a predetermined time interval subsequent to the previous ventricular beat.

As explained in the Berkovits application, if an atrial stimulating pulse is generated following anatrial'contraction, that is, during the refractory interval of the atria, it has no effect on the beating action of the patients heart. It is only the generation of a ventricular stimulating pulse during the refractory interval of the ventricles that can be dangerous. For this reason, the,

detection of a spontaneous ventricular beat inhibits the generation of the ventricular stimulating pulse which would otherwise soon occur. But atrial stimulating pulses are not inhibited. In fact, even ifv the heart beats perfectly,'in the conventional type atrial-and ventricular pacer, an atrial stimulating pulse is generated during every heartbeat cycle. Of course, the timing of the atrial pulsegenerator is keyed to the ventricular beats so that at all times both pulsers and the natural heart activity remain in synchronism.

The Berkovits pacer is provided with two timing circuits. Typical escape intervals are 600 milliseconds for the atrial timing circuit and 800 milliseconds for the ventricular timing circuit. Each ventricular beat results in the resetting of both timing circuits. After 600 milliseconds have elapsed, an atrial stimulating pulse is generated. If a ventricular beat is not detected within the next 200 milliseconds, the ventricular timing circuit causes a ventricular stimulating pulse to be generated. If a spontaneous ventricular beat is detected before 800 milliseconds have elapsed subsequent to the previous beat, then both timing circuits are reset; the result is that a ventricular stimulating pulse is not generated and a new cycle is initiated. The time period between the atrial and ventricular stimulating pulses is known as the AV delay, and in the illustrative example is 200 milliseconds.

Both escape intervals are usually adjustable; thus the AVdelay, which is the difference between them, is similarly adjustable. In the Berkovits pacer, both timing periods are controlled by charging capacitors. Each capacitor is discharged upon the detection of a spontaneous ventricular beat or the generation of a ventricular stimulating pulse. Each capacitor then starts to charge toward a respective firing level. The atrial capacitor reaches the atrial firing level after 600 milliseconds have elapsed and the ventricular charging'capacitor extended to the exterior of the pacer and access may 7 be gained to them by using a conventional Keith needle.

For maximum cardiac output, there is an optimum AV delay for each patient. The optimum AV delay is usually a fixed parameter for the patient. That is, even if the pacer rate is changed by the physicial the AV delay should generally remain the same. However, there are situations in which'the AV delay must be changed. It is apparent that by varying the resistances in the two charging circuits, both the rate and the AV,

delay may be adjusted. For example, suppose that is is necessary to increase the pacer rate. This can be achieved by decreasing the resistance in the ventricular charging circuit so that the ventricular escape interval is shortened. If the AV delay is to be maintained constant, the atrial charging resistance also should be decreased so that the atrial escape interval is similarly shortened; in this manner, an atrial stimulating pulse will still be generated the same fixed time interval prior to the generation of a ventricular stimulating pulse.

It isusually necessary for the physician to manipulate both potentiometers when making an adjustment. Suppose that .the initial escape intervals are 600 and 800 milliseconds respectively,-and that the ventricular escape interval is to be decreased by 50 milliseconds with no change in the AV delay. It is theoretically possible to gang the two potentiometers to each other so that adjustment of the ventricular potentiometer results in the automatic adjustment of the atrial potentiometer. However, it is apparent that a reduction in the ventricular escape interval from 800,milliseconds tov v750 milliseconds is a change of 6.25 percent while the change in the atrial escape interval from 600 milliseconds to 550'milli seconds is a change of 8.33 percent. It is thus difficult to provide for proper tracking of the potentiometers, and even when a fixed AV delay is'desired it is usually necessary for the physician to adjust both potentiometers when he changes the pacer rate.

Because the two potentiometer controls are not completely independent, two problems are presented. The first is that it is difficult and time consuming for the physician to properly set both potentiometers. The sec- 0nd problem is even more severe. It is possible for the physician to mistakenly set the atrial escape interval to so short a time period that two and even three atrial stimulating pulses occur during the overall ventricular escape interval (heartbeat cycle). Multiple atrial stimulating pulses of this type of course are not desirable.

It is a general object of our invention to provide an atrial and ventricular demand pacer in. which two completely independent controls are provided for adjusting the pacer rate and the AV delay, and which cannot generate multiple atrial stimulating pulses. during a heartbeat cycle. With the pacer of our invention, the physician can adjust one potentiometer to change the pacer rate without affecting the AV delay, and/or he can adjust another potentiometer to change the AV delay without affecting the pacer rate. Furthermore, he cannot erroneously adjust the two potentiometers such that more than one atrial stimulating pulse is generated for each ventricular stimulating pulse.

In accordance with the principles of our invention, a first potentiometer, the basic rate control element, is in the atrial timing circuit, rather than in the ventricular timing circuit as in the prior art. Each atrial stimulating pulse triggers two one-shot multivibrators. The first multivibrator in the illustrative embodiment of the invention has a variable pulse period of 50-250 milliseconds. The second potentiometer control adjusts the period of this multivibrator. The period of the multivibrator is equal to the desired AV delay. At the trailing edge of the multivibrator pulse, a ventricular stimulating pulse is generated. However, a spontaneous ventricular beat is detected, the ventricular stimulator is inhibited so that a ventricular stimulating pulse is not generated. It is thus apparent that each atrial stimulating pulse results in a single ventricular stimulating pulse if one is necessary after the pre-set AV delay.

The second one-shot multivibrator has a period of 260 milliseconds. At the trailing edge of the multivibrator pulse, the atrial timing circuit is reset. Consequently, even though the atrial charging capacitor starts to re-charge with the generation of an'atrial stimulating pulse, thecapacitor is discharged 260 milliseconds after the atrial stimulating pulse is generated. it is only when the capacitor starts to charge once again at this time that it is allowed to charge all the way up to the atrial firing level. Even if a spontaneous ventricular beat occurs, since it occurs after the atrial stimulating pulse by at most 250 milliseconds, the atrial charging capacitor is always discharged 260 milliseconds after an atrial stimulating pulse is generated and it is only at this time that it is allowed to start charging all the way up to the atrial firing level.

Suppose, for example, that it is desired that the pacer rate be 75 beatsper minute corresponding to an interbeat interval of 800 milliseconds. In such a case, the rate control potentiometer should be set such that the atrial charging capacitor reaches the atrial firing level 540 milliseconds after its initial discharge. Each atrial stimulating pulse triggers the 260-millisecond one-shot multivibrator. At the same time, the atrial charging capacitor is discharged and starts to charge towards the firing level. But 260 milliseconds later (by which time it has not reached the firing level) the capacitor is discharged and it starts to charge once again. After another 540 milliseconds have elapsed, the firing level is reached and another atrial stimulating pulse is generated. Consequently, atrial stimulating pulses are generated at SOD-millisecond intervals. Each atrial stimulating pulse triggers the variable one-shot multivibrator and consequently a ventricular stimulating pulse is gen erated following the pre-set AV delay after each atrial stimulating pulse provided that a spontaneous ventricular beat is not detected and the ventricular stimulater is not inhibited. Although the physician, as he adjusts the atrial timing circuit potentiometer while looking at the ECG signal on an oscilloscope, is in reality adjusting the atrial escape interval, what he observes are the large ventricular stimulating pulses; as far as the physician is concerned, he is adjusting the ventricular pulse rate. Of course, adjusting one of the. atrial .and ventricular pulse rates is equivalent to adjusting the other since for each atrial pulse there should be one ventricular pulse (in the absence of a spontaneous ventricular beat) with the two pulses separated by a fixed time interval. Since physicians are more likely to think in terms of the ventricular pulse rate, they can be informed that the atrial charging capacitor potentiometer control is the ventricular rate control. It is apparent, however, that the two controls are completely independent of each other. The rate control. potentiometer controls the rate at which both atrial and ventricular pulses occur. The AV delay control determines the time which elapses between each atrial pulse and the next ventricular pulse.

It is important to understand the function of the 260- millisecond multivibrator in the illustrative embodiment of the invention. In the pacer art, when reference is-made to rate adjustment it is conventional to think in terms of varying the ventricular escape interval. But if the ventricular escape interval is varied, the only way to keep the AV delay constant would appear to involve adjusting the atrial escape interval so that the difference between the two escape intervals is constant. This requires working backwards in time whenever a ventricular stimulating pulse occurs, the atrial stimulating pulse should have occurred a fixed time interval earlier. This is obviously a difficult thing to accomplish. However, by keying the pacer rate to the atrial charging circuit, and using a variable delay to control the AV delay, it is possible to make the rate and AV delay controls independent of each other. The difficulty with this approach, however, is that if the detection of a spontaneous ventricular beat, or the generation of a ventricular stimulating pulse, is used to synchronize the atrial charging circuit, then the basic pacer rate would necessarily be a function of the AV delay or the time when a spontaneous ventricular beat occurs. For example, witha short AV delay the period between successive atrial pulses would be shortened. However, by providing the 260-millisecond delay, the atrial charging capacitor is always discharged (synchronized) at a fixed time after any possible ventricular stimulating pulse or spontaneous ventricular beat. Consequently, the basic pacer rate can be made independent of spontaneous ventricular beats and ventricular stimulating pulses simply by controlling the atrial escape interval to be equal to the difference between the desired inter-beat interval and the delay of the 260-millisecond one-shot multivibrator.

It is a feature of our invention to provide an atrial timing circuit which is always re-started a fixed time interval after each atrial stimulating pulse and which includes a variable element for determining the basic pacer rate, the atrial timing circuit being re-started in all normal cases after the detection of a spontaneous ventricular beat or the generation of a ventricular stimulating pulse.

It is another feature of our invention to provide a variable delay for controlling the generation of a ventricular stimulating pulse at a selected time after the generation of an atrial stimulating pulse, the detection of a spontaneous ventriculating beat inhibiting the generation of a ventricular stimulating pulse.

Further objects, features and advantages of our invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts schematically the atrial and ventricular demand pacer disclosed in the above-identified Berkovits application; and 4 FIG. 2 depicts schematically the atrial and ventricular demand pacer of our invention, the design of the individual circuits shownas blocks in the drawing being apparent to those skilled in the-art especially in view of the detailed circuitry disclosed in the Berkovits application.

H6. 1 depicts three circuit blocks, the details of which are disclosed in the Berkovits application. Ventricular demand stimulator 12 applies a ventricular stimulating pulse, on ventricular electrodes El and E2, at a time after the previous ventricular beat, whether spontaneous or stimulated, which is determined by the setting of potentiometer 18. Typically, the ventricular escape interval might be 800 milliseconds. Since potentiometer 18 determines the basic pacer rate by varying the interval between ventricular stimulating pulses, it is labelled R-R rate control because the R waves in an ECG signal correspond to ventricular beats. A ventricular beat also results in the appearance of a pulse on the electrodes and conductors 22, 24 which is extended to ventricular beat detector 14. The ventricular beat detector extends a signal over conductor 16 to the ventricular demand stimulator, in response to the detection of a ventricular beat, to inhibit the generation of the next ventricular pulse which otherwise would be generated and to control the start of a new ventricular timing interval. Atrial demand stimulator is provided to extend an atrial stimulating pulse to electrodes E3 and E4. The atrial stimulator timing period starts together with the ventricular stimulator timing period, the ventricular beat detector output being extended to both pulse generators. However, the timing period of the atrial stimulator is, for example, 600 milliseconds 7 rather than 800 milliseconds filheatrial escape interval is determined by the setting of potentiometer 20. Since atrial beats correspond to P waves in an ECG signal and potentiometer determines the interval between a ventricular beat and an atrial beat, it is labelled R-P control.

In the circuit of FIG. 1, as shown in detail in the Berkovits application, two capacitors are provided to control the atrial timing period and the ventricular timing period. Each capacitor charges from an initial value toward a respective firing level. If potentiometer 20 is set to provide an atrial escape interval of 600 milliseconds, then it takes 600 milliseconds for the atrial capacitor to reach the atrial firing level, at which time an atrial stimulating pulse is generated, the capacitor is discharged and a new timing interval begins; if potentiometer 18 is set to provide a ventricular escape interval of 800 milliseconds, it requires 800 milliseconds for the ventricular timing capacitor to reach the ventricular firing level, at which time a ventricular stimulating pulse is generated, the capacitor is discharged and a new ventricular timing period begins. Both capacitors are discharged and new. timingperiods begin when either a spontaneous ventricular beat is detected ora ventricular stimulating pulse is generated.

Atrial'demand stimulator 10 in FIG. 2 is similar to atrial demand stimulator 10 in H6. 1. However, potentiometer 20 controls the basic pacer rate and accordingly is labelled P-P rate control. The atrial timing period begins when the resetinput of atrial demand stimulator 10 is pulsed. An atrial charging capacitor (as disclosed in theBerkovits application) starts to charge and when it reaches the atrial firing level a stimulating pulse is generated on atrial electrodes E3 and E4. At the same time, a pulse is applied on conductor 40 to the trigger input of each of 260-millisecond one-shot multivibrator and 50-250 millisecond one-shot multivibrator 32. Multivibrator 30 applies a short pulse at its output 260 milliseconds after it is triggered. The pulse is extended through OR gate 38 to the reset input of atrial demand stimulator 10. It requires longer than 260 milliseconds for the charging capacitor in atrial demand stimulator 10 to reach the firinglevel. Accordingly, following the generation of each atrial stimulating pulse, the capacitor starts to rise toward the firing level, but it is always discharged 260 milliseconds into the timing period before it has reached the charging level. It then starts to charge once again.

Potentiometer 34 is adjusted in accordance with the desired AV delay. For example, suppose that the desired AV delay is 200 milliseconds. In such a case, when conductor 40 is pulsed together with the generation of an atrial stimulating pulse, multivibrator 32 is triggered. The multivibrator pulses its output 200 milliseconds after it is triggered. Ventricular pulser 36 simply serves to amplify the pulse and to apply a ventricular stimulating pulse to electrodes El and E2, provided it is not inhibited from operating by a reset pulse on conductor 16. Consequently, in the absence of the'detection of a spontaneous ventricular beat, a ventricular stimulating pulse is applied on electrodes El and E2 at a time after the atrial stimulating pulse is generated in accordance with the AV delay setting of potentiometer 34. I

If during the AV delay interval a spontaneous ventricular beat occurs, the electrical signal on conductors 22, 24 is detected'by ventricular beat detector 14. Conductor 16 is pulsed and the reset input of ventricular stimulator 36 is energized. The stimulator is reset so that a ventricular stimulating pulse is not generated at the end-of the pulse generated by one-shot multivibra: tor 32. Conductor 16. is also connected to an input 'of OR gate 38 so that the detection of a ventricular beat re-starts the atrial timing period.

The 260-millisecond period of one-shot multivibrator 30 is made longer than the maximum AV delay interval. In the event aspontaneous ventricular beat occurs before multivibrator 32 completes its timing period, ventricular stimulator 36 is inhibited so that a ventricular stimulating pulse is not generated, OR gate 38 operates and the charging capacitor in atrial demand stimulater 10 is discharged. The capacitor starts to charge once again, but it is again discharged at the trailing edge of the 260-millisecond pulse output of multivibrator 30. On the other hand, if a spontaneous ventricular beat does not occur, then with the expiration of the pulse period of multivibrator 32, a ventricular stimulating pulse is generated. This similarly results in the operation of ventricular beat detector 14 and the resetting of atrial demand stimulator 10. The atrial charging capacitor starts to charge but is discharged in this case also at the end of the 260-millisecond timing interval.

No matter what the ventricular activity under normal circumstances, multivibrator 30 always controls the resetting of the atrial demand stimulator 260 milliseconds after the previous atrial stimulating pulse. Consequently, the effective atrial timing period, that is, the charging cycle of the capacitor which ultimately results in the reaching of the firing level, always begins 260 milliseconds after a previous atrial stimulating pulse.

To achieve any desired basic pacer rate, all that is necessary is to adjust potentiometer 20 so that the escape interval of atrial demand stimulator 10 is the desired inter-beat interval minus 260 milliseconds. Potentiometer 20 thus controls the basic pacer rate because a ventricular stimulating pulse, if one is needed, always occurs after an atrial stimulating pulse in accordance with the setting of potentiometer 34, and the time separation between ventricular stimulating pulses is necessarily the same as the time spacing between atrial stimulating pulses. As far as the physician is concerned, potentiometer 20 controls the basic (ventricular) pacer rate, although in reality potentiometer 20 directly affects the time interval between successive atrial stimulating pulses.

The reason for one-shot multivibrator 30 is as follows: No matter when a'ventricular beat takes place within the 260 milliseconds following the generation of an atrial stimulating pulse, even though that ventricular beat results'in the resetting of the atrial demand stimulator, it does not affect the atrial stimulating pulse timing. Instead, the atrial stimulating pulse timing is fixed in accordance with the setting of potentiometer 20 because multivibrator 30 always re-starts the atrial escape interval no matter when ventricular beat detector 14 pulses its output between the generation of an atrial stimulating pulse and the completion of the timing in- 'terval of one-shot multivibrator 30. As for the AV delay, it is keyed to the generation of the atrial stimulating pulses, with aventricular stimulating pulse, if one is necessary, being generated a pre-set time after the generation of an atrial stimulating pulse. The two controls are completely independent of each other. Varying the setting of potentiometer 34 does not'affect the basic pacer rate and instead only results in a change in the AV delay. Similarly, a change in the setting of potentiometer 20 changes the basic pacer rate; a ventricular stimulating pulse, if one is necessary is always generated at a fixed time after the generation of an atrial stimulating pulse in accordance with the setting of potentiometer 34.

The output of ventricular beat detector 14 resets the atrial demand stimulator during normal operation even though such resetting has no effect because the atrial demand stimulator is always reset once again when multivibrator 30 completes its timing interval. The reason for allowing the ventricular beat detector to reset the atrial demand stimulator is to take into account the possibility of an abnormal ventricular beat which occurs more than 260 milliseconds after the generation of an atrial stimulating pulse. For example, it is possible for one-shot multivibrator 32 to stimulate a ventricular beat, and that some time shortly thereafter a spontaneous ventricular beat Occurs before the next atrial stimulating pulse. In order to key the pacer operation to the last ventricular beat, ventricular beat detector 14 causes the atrial timing period to start all over again.

It should be noted that although the pacers of FIGS. 1 and 2 are for the most part functionally equivalent, there is a situation in which the functional sequences of the two pacers are different. Suppose, for example, that the inter-beat interval is adjusted for 800 milliseconds (that is, the atrial escape interval in the pacer of FIG. 2 is 540 milliseconds), and that the AV delay is set to 200 milliseconds. Suppose further that a spontaneous ventricular beat occurs milliseconds after the generation of an atrial stimulating pulse. In the pacer of FIG. 1, the spontaneous ventricular beat results in the resetting of both the atrial demand stimulator and the ventricular demand stimulator. Consequently, the next atrial stimulating pulse will occur 600 milliseconds after the spontaneous ventricular beat, and the next ventricular stimulating pulse, if one is necessary, will occur 800 milliseconds after the spontaneous ventricular beat. However, in the pacer of FIG. 2, the detection of the spontaneous ventricular beat has no effect. Even though atrial demand stimulator 10 is reset 100 milliseconds after the generation of the previous atrial stimulating pulse, multivibrator 30 causes the atrial demand stimulator to be reset once again milliseconds later. In effect, as far as the atrial timing is concerned, the pacer ignores the fact that a spontaneous ventricular beat occurred 100 milliseconds before a ventricular stimulating pulse would have otherwise been generated. The extra approximately IOU-millisecond delay before the next atrial stimulating pulse is generated provides for a built-in compensatory pause. By building in this delay, it is more likely that a spontaneous atrial heat will occur, followed by a spontaneous ventricular beat, than in the case of the pacer of FIG. 1 in which the normal timing of the pacer ensues without any compensation being made for the fact that a premature ventricular beat occurred. Of course, if a spontaneous ventricular beat does occur before the atrial demand stimulator completes its timing interval, then battery life is extended because even an atrial stimulating pulse is not generated during the next cycle. It is emphasized, however, that the major advantage of the pacer of FIG. 2 is believed to be the independent'control of the basic pacer rate and the AV delay, and also the prevention of the possibility of the controls being adjusted such that more than one atrial stimulating pulse occurs between successive ventricular stimulating pulses.

Ventricular stimulator 36 operates to generate a ventricular stimulating pulse at the trailing edge of the pulse 'output of multivibrator 32, but as described above a stimulating pulse is not generated if the reset input of the ventricular stimulator is energized. Many different circuits can be used for this purpose. A preferred circuit is one which is very similar to the basic timing circuit included in a demand stimulator. In this circuit, the output of one-shot multivibrator 32 is capacitor coupled to the reference potential terminal. The reference potential is normally high. The output of the multivibrator is a positive pulse, and consequently at its trailing edge a negative spike is coupled through the capacitor to the reference potential terminal. Only at this time is the reference potential to which the timing capacitor voltage is compared lowered sufficiently such that the timing capacitor voltage exceeds it and causes a stimulating pulse to be generated. However, a reset pulse generated by ventricular beat detector 14 causes the timing capacitor to discharge (just as it is discharged in the circuit of FIG. 1). The time constant of the charging circuit is adjusted such that it requires more than 250 milliseconds for the capacitor voltage to rise to a level which exceeds even the low reference potential which results at the trailing edge of the output pulse of multivibrator 32. Consequently, once a reset pulse has discharged the charging capacitor in the ventricular stimulator, a stimulating pulse cannot be generated, even at the trailing'edge of the multivibrator pulse.

Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles-of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

What we claim is:

1. A pacer comprising atrial terminal means, ventricular terminal means, first timing means for applying a pulse to said atrial terminal means at the end of a first time interval following the resetting thereof, means for varying said first time interval, second timing means for applying a pulse to said ventricular terminal means at the end of a second time interval following the application of a pulse to said atrial terminal means, means for adjusting said second time interval, means for detecting a ventricular beat of a-patients heart and responsive thereto for inhibiting the application of a pulse to said ventricular terminal means if said second time interval has not expired subsequent to the application ofthe previous pulse to said atrial terminal means and for resetting said first timing means, and means for resetting said first timing means at the end of a third time interval following the application of a pulse to said atrial terminal means, said third time interval being longer than the maximum second time interval.

2. A pacer in accordance with claim 1 wherein said second time interval is adjustable within minimum and maximum time intervals defining a normal range of AV delays.

3. A pacer comprising atrial terminal means, ventricular terminal means, means for generating atrial stimulating pulses on said atrial terminal means at spaced time intervals, means for generating a ventricular stimulating pulse on said ventricular terminal means at the end of a selected time interval following the generation of each stimulating pulse on said atrial terminal means, means for establishing the maximum value of said selected time interval to be less than'a predetermined interval between the generation of said atrial stimulating pulse and the start of the next said spaced interval, means for detecting a spontaneous ventricular beat of a patients heart within said selected time interval following the generation of a stimulating pulse on said atrial terminal'means but prior to the generation of a corresponding stimulating pulse on said ventricular terminal means for inhibiting the generation of said stimulating pulse on said ventricular terminal means, and means for controlling the time interval between a spontaneous ventricular beat and the generation of the next stimulating pulse on said atrial terminal means to be greater than the time interval between the generation of a stimulating pulse on said ventricular terminal means and the generation of the next stimulating pulse on said atrial terminal means.

4. A pacer in accordance with claim 3 wherein the time difference between the time interval separating a spontaneous ventricular beat and the next stimulating pulse on said atrial terminal means and the time interval separating a stimulating pulse on said ventricular terminal means and the next stimulating pulse on said atrial terminal means corresponds to the compensatory pause which follows a premature beat of said patients heart.

5. A pacer in accordance with claim 3 wherein said atrial stimulating pulse generating means includes another means for generating an atrial stimulating pulse at the end of another selected time interval following the detection of a spontaneous premature ventricular beat or following the lapse of a predetermined time interval after the generation of a previous atrial stimulating pulse, whichever occurred later.

6. A pacer in accordance with claim 5 further including a pair of means each for adjusting one and only one of a respective one of said selected-time interval and said other selected time interval.

7. A pacer in accordance with claim 6 including means for establishing said predetermined time interval longer than the maximum value of said selected time interval. 

1. A pacer comprising atrial terminal means, ventricular terminal means, first timing means for applying a pulse to said atrial terminal means at the end of a first time interval following the resetting thereof, means for varying said first time interval, second timing means for applying a pulse to said ventricular terminal means at the end of a second time interval following the application of a pulse to said atrial terminal means, means for adjusting said second time interval, means for detecting a ventricular beat of a patient''s heart and responsive thereto for inhibiting the application of a pulse to said ventricuLar terminal means if said second time interval has not expired subsequent to the application of the previous pulse to said atrial terminal means and for resetting said first timing means, and means for resetting said first timing means at the end of a third time interval following the application of a pulse to said atrial terminal means, said third time interval being longer than the maximum second time interval.
 2. A pacer in accordance with claim 1 wherein said second time interval is adjustable within minimum and maximum time intervals defining a normal range of AV delays.
 3. A pacer comprising atrial terminal means, ventricular terminal means, means for generating atrial stimulating pulses on said atrial terminal means at spaced time intervals, means for generating a ventricular stimulating pulse on said ventricular terminal means at the end of a selected time interval following the generation of each stimulating pulse on said atrial terminal means, means for establishing the maximum value of said selected time interval to be less than a predetermined interval between the generation of said atrial stimulating pulse and the start of the next said spaced interval, means for detecting a spontaneous ventricular beat of a patient''s heart within said selected time interval following the generation of a stimulating pulse on said atrial terminal means but prior to the generation of a corresponding stimulating pulse on said ventricular terminal means for inhibiting the generation of said stimulating pulse on said ventricular terminal means, and means for controlling the time interval between a spontaneous ventricular beat and the generation of the next stimulating pulse on said atrial terminal means to be greater than the time interval between the generation of a stimulating pulse on said ventricular terminal means and the generation of the next stimulating pulse on said atrial terminal means.
 4. A pacer in accordance with claim 3 wherein the time difference between the time interval separating a spontaneous ventricular beat and the next stimulating pulse on said atrial terminal means and the time interval separating a stimulating pulse on said ventricular terminal means and the next stimulating pulse on said atrial terminal means corresponds to the compensatory pause which follows a premature beat of said patient''s heart.
 5. A pacer in accordance with claim 3 wherein said atrial stimulating pulse generating means includes another means for generating an atrial stimulating pulse at the end of another selected time interval following the detection of a spontaneous premature ventricular beat or following the lapse of a predetermined time interval after the generation of a previous atrial stimulating pulse, whichever occurred later.
 6. A pacer in accordance with claim 5 further including a pair of means each for adjusting one and only one of a respective one of said selected time interval and said other selected time interval.
 7. A pacer in accordance with claim 6 including means for establishing said predetermined time interval longer than the maximum value of said selected time interval. 